An electret is a solid dielectric that exhibits persistent dielectric polarization. In particular, an electret is a solid dielectric with a quasi-permanent electric moment. Electrets may be classified as real-charge electrets and dipolar-charge electrets. Real-charge electrets are dielectrics with charges of one polarity at or near one side of the dielectric and charges of opposite polarity at or near the other side, while dipolar-charge electrets are dielectrics with aligned dipolar charges. Some dielectrics are capable of storing both real and dipolar charges.
U.S. Pat. No. 4,046,704 issued to I. Sumita in 1977 discloses a high polymer electret comprising one member of poly-3,3-bis(chloromethyl)oxacyclobutane, poly-3,3-bis-(fluoromethyl)oxacyclobutane and poly-3,3-bis(bromoethyl)oxacyclobutane group. It possesses stability. A disadvantage of this material is that stability of this material is unpredictable because of its multicomponent structure. Another drawback of this multicomponent material is strong dependence on properties of the components.
Also known in the art is a method of preparation of a polymer electret described in U.S. Pat. No. 4,291,245 issued T. Nowlin, et al., in 1981. The method comprises the following steps: (a) providing a parylene film having one side affixed to a metal layer and grounding said metal layer; (b) charging the free side of the film with a direct current corona, the charge being of sufficient magnitude to convert the film to an electret; (c) providing p-xylylene monomer vapor in sufficient amount to coat the charged film; and (d) introducing the vapor from step (c) and the electret into a deposition zone, said zone being under vacuum and at a temperature at which the vapor will condense, whereby the electret is conformally coated with parylene. Its disadvantages include complex technology and multistage preparing, as well as existing unstable low organic compounds in the composition that reduce the life of the electret.
Known in the art is another polymer electret described in U.S. Pat. No. 4,626,263 to N. Inoue, et al., in 1981. The electret comprises 60 to 99% of non-polar polymer, 0.5 to 39.5% of polar polymer, and 0.5 to 20% of at least one component selected from the group consisting of (A) a non-polar polymer modified with an unsaturated carboxylic acid or a derivative thereof, (B) a non-polar polymer modified with an unsaturated epoxy monomer and (C) a non-polar polymer modified with a silane monomer having an olefinically unsaturated bond. In this electret, a high charge density can be maintained stably over a long period, and this electret can be easily formed into a film. An air filter prepared from this electret has excellent dust collecting efficiency. Its disadvantages include complex compounds, limited temperature range of stability, and instability in a heterocharge relaxor state caused by orientation of the polar polymer component during electrothermopolarization.
U.S. Pat. No. 6,573,205 issued to D. Myers, et al., in 2003 describes a porous polymeric sheet having an electrostatic charge and comprising a zero-three composite of a polymeric matrix and a ferroelectric material dispersed therein. The polymeric component comprises a non-polar thermoplastic polymer, such as a polyolefin, and a second thermoplastic polymer having polar functional units, such as a telomer. The composite material is formed into a porous sheet and is electrically or corona poled to create an electret material which is well suited for use in various filtration, air-masking and dust wipe applications. Its disadvantages included using barium titanate, barium titanate strontium, lead titanate and solid solutions based on their ferroelectric phase; as well as having relatively low Curie temperature and unstable domain structure.
U.S. Patent Application Publication 20080249269 (published in 2008 and invented by H. Chin, et al.) describes a polymer electret with outstanding thermal and charge stability. The electret materials comprise a melt blend of a thermoplastic polymer and one or more compounds selected from the aromatic trisamides. The aromatic trisamides are for example of the formula. The melt blends are subjected to an electret treatment, for example a corona treatment. The electret materials are for example nonwoven polyolefin webs and are employed as filter materials, wipes, absorbent materials, filter masks, acoustic materials, printing substrates, measuring devices or contactless switches. The electret materials may also comprise an additive selected from a hindered amine light stabilizers and hydroxyphenylalkylphosphonic esters or monoesters. Disadvantages of such electrets include complex manufacturing, use of low-molecular-weight aromatic materials, and unpredictability in formation of a system comprising homo- and heterocharges that define electret properties.
The known polymer electrets, however, possess a number of disadvantages such as low potential difference, relatively short lifespan, insufficient dielectric permeability and electric resistance, complexity of manufacture, and a multicomponent composition.